Nutated slice reconstruction (NSR)-type algorithms are described in U.S. Pat. No. 5,802,134, Larson et al., “Nutating slice CT image reconstruction apparatus and method,” 1999 (“Larson”); U.S. Pat. No. 7,215,805 B2, Bruder et al., “Method and apparatus for spiral scan computed tomography,” 2007 (“Bruder”); S. Schaller, K. Stierstorfer, H. Bruder, M. Kachelrietβ, and T. Flohr, “Novel approximate approach for high-quality image reconstruction in helical cone beam CT at arbitrary pitch”, Proc. SPIE vol. 4322, p. 113-127, 2001 (“Schaller”); and U.S. Pat. No. 7,027,552 B2, “High resolution CT scanner,” 2006 (“Shechter”). As described these nutated slice-type algorithms are approximate algorithms used to reconstruct helical scan data from multi-row CT scanners. The algorithms interpolate multi-row data into slices (planes) which have normals that are tilted with respect to the gantry rotation axis. The angle of the tilted planes is chosen to fit the helical path of the x-ray source in order to reduce artifacts. These tilted slices are then reconstructed using 2D filtered backprojection techniques, followed by interpolation in the direction of the rotation axis (“untilting”) to produce images perpendicular to the rotation axis.
Each tilted slice touches the helical trajectory of the x-ray source at a locus called “attachment point.” The original NSR algorithm described by Larson uses a single tilted plane at each attachment point which was suitable for higher helical pitch applications. This concept was extended in the Adaptive Multiple Plane Reconstruction (“AMPR”) algorithm described in Bruder and Schaller. The AMPR algorithm uses multiple tilted planes at each attachment point to improve dose utilization for lower pitches. In CT scanners with an x-ray focal spot which is fixed with respect to the rotating detector array, the NSR and AMPR algorithms use of half-scan reconstruction approach results in the in-plane sampling rate of 1/wdiso near isocenter, where wdiso is the in-plane pitch of the detector at isocenter. This sampling rate limits the resolution of the reconstructed images. This type of focal spot shall be henceforth referred to as “fixed focal spot.”. One approach to improve the resolution of CT scanner in an axial scanning mode is to use interleaving of rays which are 180 degrees apart and offset by a quarter detector, as described in U.S. Pat. No. 4,051,379 Zacher, Jr. “Axial tomography apparatus and detector” 1977 (“Zacher, Jr.”). Another approach to improve the resolution of a CT scanner in helical and axial scanning modes is to increase the in-plane sampling rate by using an x-ray source with a “flying focal spot,” as described in U.S. Pat. No. 6,256,369 Lai, “Computerized tomography scanner with longitudinal flying focal spot” 2001 (“Lai”) which may be more expensive than a fixed focal spot source.